The settling behavior of amplifier circuit consists of two distinct modes of operation. Initially, the circuit is in a slewing mode, and then it goes into a small signal mode. Thus, the total setting time of the circuit (STtotal) can be determined based on the time spent for the slewing mode (STslew) and the time spent for settling the small signal mode (STsmall-signal), i.e., STtotal=STslew+STsmall-signal. The proportion between STslew and STsmall-signal depends on many factors such as fabrication process used for the circuit components, amount of capacitance each amplifier is driving, the accuracy required for the circuit, and so on. Furthermore, STslew=K1*I−1bias and STsmall-signal=K3*I−0.5bias*ν−0.5=K2*I−0.5bias*T0.75 where T is the temperature, K1, K2 and K3 are temperature independent constants, Ibias is the bias current for the circuit, and ν is the mobility of electrons which is temperature dependant.
Proportional to absolute temperature (PTAT) bias current is often used to maintain the settling behavior of amplifier circuit at hot temperatures. When the temperature goes up from the reference temperature (e.g., room temperature), the circuit slows down because of the slow down of electrons in high or hot temperatures. The PTAT current is used as Ibias to compensate for the slowdown of electrons since the PTAT current increases with the temperature rise.
STsmall-signal is maintained over a range of temperature since the bias current (e.g., I−1bias) is compensated by temperature (e.g., T0.75) as described in the equation. STslew, on the other hand, decreases as the PTAT current used as Ibias increases with the rise of temperature. However, when the temperature falls below the reference temperature, STslew increases as the PTAT current used as Ibias decreases. As a result, the degraded settling behavior limits the performance of high speed amplifier circuits at cold or low temperatures.